ZSM-5 and its synthesis using tetrapropylammonium (TPA) cations as a directing agent are disclosed in U.S. Pat. No. 3,702,886. U.S. Pat. No. 3,926,782 discloses hydrocarbon conversion over ZSM-5 crystals having a crystal size of 0.005-0.1 micron synthesized in the presence of TPA cations.
U.S. Pat. No. 4,151,189 discloses that ZSM-5 can be synthesized in the presence of a primary amine having 2-9 carbon atoms, particularly n-propylamine. U.S. Pat. No. 5,369,071 discloses the use of n-propylamine in the synthesis of ZSM-5 with a silica to alumina ratio as low as 20.3 from a reaction mixture having a pH 10-14, an OH-/SiO.sub.2 ratio of 0.1-0.3, an M/SiO.sub.2 ratio of 0.2-0.6 (where M is an alkali or alkaline earth metal) and an H.sub.2 O/SiO.sub.2 ratio of 10-35.
EP-A-106552 teaches that ZSM-5 and ferrierite can be synthesized in the absence of an inorganic directing agent by using an amorphous granular silica-alumina as the source of silicon and aluminum. The resultant ZSM-5 is said to have a silica to alumina molar ratio of 15-100 but the only ZSM-5 product exemplified has a silica to alumina molar ratio of 58.8. EP-A-106552 fails to disclose the crystal size of the ZSM-5 produced.
EP-A-306238 discloses that ZSM-5 crystals having a platelet morphology with two dimensions of at least 0.05 micron, typically at least 0.1 micron, and a third dimension less than 0.02 micron can be synthesized from a non-organic synthesis mixture having at least 35 wt % solids and an OH-/SiO.sub.2 ratio of at least 0.11.
Other non-organic synthesis routes for ZSM-5 are known and commercially practiced and typically produce a material having a silica to alumina molar ratio of 20-30 and a crystal size of about 0.2.0.5 micron.
To date it has proved extremely difficult to produce ZSM-5 from reaction mixtures with silica to alumina molar ratios less than about 20, which could produce crystals with correspondingly lower framework silica to alumina molar ratio. Framework aluminum sites are responsible for the acid activity of zeolites, and it is desirable for many catalytic uses to be able to produce ZSM-5 with a framework silica to alumina molar ratio as low as possible. Similarly, for catalytic uses where rapid diffusion of reactants and products into and out of the zeolite is desirable, it is important to be able to produce ZSM-5 with a small crystal size, for example less than 0.1 micron.
The problem of producing ZSM-5 with a low silica to alumina molar ratio has been particularly pronounced in the case of small crystal materials. Thus to date small crystal ZSM-5, with a crystal size of less than 0.1 micron, has been obtained only with silica/alumina ratios higher than approximately 23:1.
The crystal size of a zeolite can be determined by direct measurement using electron microscopy. However, other indirect methods of determining crystal size are available and can be useful in differentiating between small crystal materials, especially when no exact size can be assigned visually as the result of size polydispersity, irregular/non-uniform shape and/or extensive crystal intergrowth. For example, the nitrogen adsorption/desorption isotherm showing the amount of nitrogen adsorbed by a solid at 77.degree. K as the function of relative partioal pressure p/p.sub.0 can be used to gauge and compare average crystal size of materials. The isotherm can be used to calculate apparent internal (zeolite) and external (mesoporous) surface area, ZSA and MSA, respectively, of the crystals. Increasing MSA indicates decreasing crystal size. At low nitrogen partial pressures the isotherm tracks filling of the zeolite micropores but at higher relative partial pressures, i.e. 0.4-0.7 for ZSM-5, the slope of the isotherm reflects the crystal size. This latter approach is useful when ambiguity in determining the MSA/ZSA split may arise.
According to the invention, a novel form of ZSM-5 has now been produced with a combination of an unusually low silica to alumina molar ratio and a very small crystal size.
It is to be appreciated that, although ZSM-5 is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron, iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.